1. Field of Invention
The present invention is related to components useful in securing internal and external orthopedic fixation devices, particularly rod-based internal spinal fixation systems.
2. Description of Related Art
Internal fixation of the spine has been important in the surgical management of a variety of spinal disorders. The vast majority of implants for this purpose employ a longitudinal stabilizing device (either a rod or a plate) which is attached to the spine by means of hooks or screws. Hooks typically engage the spine by their position of insertion around the spinal lamina or transverse process. Screws engage the spine by affixing directly to the bone of the anterior vertebral body or through the pedicles posteriorly. An additional option in the cervical spine is the insertion of screws into the lateral masses of the vertebrae. Attachment of the hook and/or screw to the longitudinal fixation device is an important linkage in this construct.
The spinal surgeon would ideally wish to have a means to achieve an extremely rigid stabilization of the hook or the screw to the longitudinal fixation device. There are also significant advantages to coupling systems which allow the hook or screw to be affixed to the longitudinal fixation device through a variety of different angles.
One of the original devices for internal fixation of the spine is the Harrington rod. This device consists of a stainless steel rod to which is affixed hooks designed to be placed beneath the laminae of the vertebrae posteriorly. A single hook is attached to the rod inferiorly by means of a hole through the top of the hook into which the rod rests. The second hook also contains such a hole which engages the rod in a ratchet-like mechanism to allow the application of distraction. Numerous spinal fixation devices since the initial development of the Harrington rod have employed the technique of threading the rod through a hole in the screw or hook. Refinements in this technique includes implants such as the Cotrel-Dubosset in which multiple hooks and/or screws can be inserted along the length of a single rod. In the case of the Cotrel-Dubosset implant, this is accomplished by means of small flat-headed locking screws which transfix the cannulated hook and directly engage the rod by single point-fixation. Hooks used in the Cotrel-Dubosset system are fixed to the rods via the means of these one, two, or three small locking screws.
On some occasions (particularly during surgery for spinal deformity), it is not possible to have such closed cannulated hook and screw heads, and threading a rod through multiple hooks or screws can be difficult or impossible. To overcome this, the Cotrel-Dubosset implant system uses an "open" screw head which allows the rod to be placed into the screw head or hook head from above. In the case of the Cotrel-Dubosset device, this is accomplished by means of a circular insert which fits into the semi-circular screw head and subsequently accepts locking screws identical to those used in the closed hook and screw systems. In both "open" and "closed" hooks and screws, the rod nonetheless must be contoured to lie fairly precisely within the screw head or hook fitting in order to allow stable fixation to the rod.
A similar design is found in Acromed's ISOLA system. In this system, "open" and "closed" headed hooks and screws are also available. The "closed" head design allows passage of the rod through the hole in the screw or hook head which is then fixed in place by a single, vertically-oriented locking screw. Open head designs for the ISOLA system involve a sliding "top" to the screw or hook head which contains the locking screw. The sliding top of the screw or hook head snaps into position and allows placement of the single locking screw for fixation of the rod. This system also requires contouring of the rod through the screw head in every application.
Another approach applied in situations where threading the rod through multiple screw/hook heads is too difficult or impossible involves designs in which the rod passes adjacent to the screw or hook. Examples of this would be the Texas Scottish Rite Instrumentation (TSRI) in which the rod is fixed to the screw by the shortening action on a transverse linkage effected by tightening a bolt. A top tightening version of this instrumentation system involves an obliquely-placed top loading single locking screw. The TSRI system allows for rotation of the hook or screw in the sagittal plane prior to tightening. This increased flexibility allows considerably decreased operative time and effort in accurately contouring the rod since the rod can be rigidly attached to screws at various angles in this sagittal plane without contouring. However, when implanting the system, it is extremely important that the spinal fixation rod be properly seated within the groove of each eyebolt prior to tightening to ensure secure three-point fixation upon the rod once tightening is performed. Further, to ensure a secure grip, the bolt must be tightened down to a minimum of 150 in-lb to reduce the chance of nut loosening.
It is therefore desirable to have an internal spinal fixation device suitable for rod-based fixation in both the anterior and posterior spinal regions which further includes the following attributes:
(1) a simple, modular design employing interchangeable components; PA1 (2) requires few components; PA1 (3) requires only minimal or no rod contouring by allowing for variable angulation and adjustment of the screw or hook head in both the sagittal and frontal planes; PA1 (4) contains a locking system that securely engages the rod to prevent slippage; and PA1 (5) requires no special training to use.